Known image processing includes a binarization process from a multivalued image including pixels having density of intermediate tones into an image having black pixels and white pixels. This process determines whether a target pixel is an edge pixel, then it is determined based on the determination result whether the target pixel is a part of a line segment. When the target pixel is not a part of a line segment, the target pixel is determined to be either a black pixel or a white pixel based on an error diffusion method. When the target pixel is a part of a line segment, regardless of a result based on the error diffusion method, the target pixel is determined to be either a black pixel or a white pixel depending on the adjacent pattern. Accordingly, it is described that this process enables to prevent a part of a thin line from being broken. In addition, regardless of whether the target pixel is determined to be either a black pixel or a white pixel, the error based on the result of the error diffusion method is distributed to the adjacent pixels.